1. Field of Invention
This invention relates to electrodeposition of chromium on basis metals, and, more particularly, it is concerned with an activation process by which an adherent chromium electrodeposit is formed on ferrous metal substrates using a high energy efficient chromium plating bath.
2. Description of the Prior Art
The use of high efficiencv chromium plating processes has been hampered by the inability to obtain coatings with adequate adhesion to certain basis metals. Chromium plating baths containing halides particularly have exhibited such adhesion problems. These baths are of the type disclosed in Mitsui, J7B-33941 (September, 1978); Dillenberg U.S. Pat. No. 4,093,522, Perakh et al., U.S. Pat. No. 4,234,396; and Chessin, U.S. Pat. Nos. 4,450,550; 4,472,249 and 4,588,481. The problem has been attributed to the presence of a halide which may interfere at the initiation of deposition.
In order to obtain an adequate bond, as measured by ASTM B 571-79, with typical chromium plating solutions, such as those using a solution of chromic acid and catalysts such as sulfate, or sulfate in combination with various fluorides, it is usually necessary to reverse or anodically etch a ferrous workpiece in the plating solution, or in a separate chromium acid containing solution at a predetermined current density for a predetermined time.A table which lists the time lengths for such an etching process is found in "Metal Finishing" 80 (5) 65-8 (1982) by C. H. Peger.
Anodic chromic acid treatments for 400 stainless steel alloys and for low and high carbon steel are disclosed in "48th Metal Finishing Guidebook-Directory" 78, 188-202 (1980) by A. Logozzo. Also recommended therein are techniques for 300 stainless, for nickel alloys and for cast iron.
Brune and McEnally, in "Plating" 42, 1127-32(1955), describe the use of a magnesium sulfate-sulfuric acid anodic etch solution for preparing ferrous parts for plating. Similarly, ASTM Specification B-242-49T suggests the application of an anodic etch using a sulfuric acid solution containing sodium sulfate. ASTM B177-68 describe the use of sulfuric acid or chromic acid as activators for chromium electroplating on steel for engineering use.
Chessin, in U.S. Pat. No. 4,450,050, described an activation pretreatment for bonding high efficiency chromium electrodeposits on a metal substrate, which process is characterized bv the step of first plating the substrate metal with iron or an iron alloy from an iron salt containing bath.
Hermann, in U.S. Pat. No. 4,416,758, activates metal substrates in an aqueous alkaline cyanide containing solution using current which is periodically reversed, followed by rinsing and chromium plating.
McMullen et al., in U.S. Pat. No. 4,585,530, describes an activation process using a substantially neutral solution of an alkali metal sulfate.
It has been found that when these prior procedures are employed with high efficiency chromium plating baths for ferrous metal substrates which includes both hardened and unhardened (soft) parts, the chromium deposits do not have adecuate adhesion to the part. It is believed that the reducing conditions at the cathode at the initiation of deposition causes the halide ion in the bath to be reduced to a form which interferes with the molecular bonding of the chromium to the substrate. In any event the use of high energy efficiency (HEEF) chromium plating for ferous substrates is limited bv the problem of inadequate adhesion.
Accordingly, it is an object of this invention to provide an improved process for forming adherent chromium electrodeposits, particularly from high energy efficient baths, on ferrous metal substrates.
Another object herein is to provide an activation solution for pretreating a ferrous metal substrate before electrodepositing chromium thereon, which solution is stable over prolonged periods.
Still another object is to provide an activation solution for electrolytically etching, preferably, anodically etching, a ferrous metal substrate in preparation for electrodepositing an adherent chromium metal deposit thereon from a high energy efficient bath, where the degree of adherence of the chromium deposit is relatively independent of the kind of ferrous metal present in the substrate.
Yet another object herein is to provide adherent and smooth chromium deposits on both hardened and unhardened ferrous metal substrates from a high energy efficient chromium electroplating bath.